In many parts of the world, the construction and use of high-speed railways is growing tremendously. Network vendors and operators need to provide reliable communications via cellular networks for travelers using user equipment (“UE”) on high-speed transportation systems, such as high-speed railways.
Cellular networks are radio networks which may be distributed over a large geographical area. The geographical area is divided into “cells.” Each cell is generally served by at least one transceiver known as a “base station,” which is often fixed in location. Together, the base stations may comprise a wireless wide area network (“WWAN”). The WWAN can also be communicatively coupled with a public or private network, which may include that particular aggregation of networks commonly known as the Internet.
UEs, which may be mobile and moving, are configured to establish connections with these base stations. These connections may be established, for example, using code division multiple access (“CDMA”), Global System for Mobile Communications (“GSM”), Universal Mobile Telecommunication System (“UMTS”), or the like. Through these connections with the base stations, the UEs are able to establish voice and/or data communications with each other and other transceivers or receivers within the network or within other connected networks, including the Internet.
The problem with using a UE on high-speed transportation systems in current cellular networks can be described with reference to FIG. 1. As shown, high-speed vehicle 110 (e.g., a high-speed train) will pass through cells 1, 2, 3, and 4 of a wireless network 140, which may be a WWAN. Accordingly, the network 140 will need to coordinate handovers of UEs 120 on board the high-speed vehicle 110 between cells 1 and 2, cells 2 and 3, cells 3 and 4, and so on.
In order to provide reliable communications for travelers on the high-speed train, the network deployment must resolve at least two types of challenges:
(1) When the speed of the vehicle 110 exceeds a certain threshold (e.g., 250 kilometers per hours (km/h)), UEs on board the vehicle 110 may pass through multiple cells (e.g., cells 1 and 2) of the cellular network 140 in a very short amount of time. This may cause excessive signaling load to the network 140, and will often result in radio link failures, handover failures, and even dropped connections (e.g., dropped calls).
(2) Depending on the WWAN technology, in current networks, it can take up to six seconds for a UE to complete a handover between cells, whereas it often takes much less than six seconds for a high-speed train, at top speed, to pass through an overlapping coverage region of two cells (e.g., the overlapping region 145). In such a case, a UE on vehicle 110 cannot reliably complete handovers, resulting in a dropped connection.
One technique which partially ameliorates the problems described above is the use of a mobile relay 130 on board the high-speed vehicle 110. A mobile relay is a relay mounted on the vehicle and capable of wireless communication with macrocells 1, 2, 3, and 4, which may be donor macrocells. Mobile relay 130 is configured to wirelessly relay data between a base station and UEs. The mobile relay 130 maintains a wireless backhaul connection with the base station and hence the network 140. The mobile relay 130 also establishes point-to-multipoint (“PMP”) connectivity with the UEs 120 on board the high-speed vehicle 110. Therefore, the mobile relay 130 can provide both uplink and downlink connectivity for each of the UEs 120.
Mobile relay 130 can perform a group mobility procedure for every UE 120 on board the vehicle 110, thereby eliminating the need for individual UE handovers. This reduces the amount of handover signaling required, thereby significantly reducing the overhead on the base stations involved. However, while the use of mobile relay 130 improves handover success rate by reducing the amount of handover signaling required, the group mobility procedure requires that mobile relay 130 itself undergoes handovers between the base stations of cells 1, 2, 3, and 4. Thus, due to the high speed of vehicle 110, mobile relay 130 will itself experience handovers at an excessive rate, resulting in dropped connections for at least the same reasons as described above in relation to individual UEs. This has resulted in the current state of communications on high-speed railways falling short of travelers' demands.
The disclosed embodiments solve the above problems by reducing the complexity of mobile relay communications. Specifically, the disclosed embodiments provide for handovers from cell to cell (or referred to herein as point-to-point) which are transparent to the mobile relay. While, the systems and methods disclosed herein will largely be described with reference to high-speed railways, it should be appreciated that the disclosed embodiments are just as applicable to other high-speed transportation systems with trackable paths, and are not limited to high-speed trains.